Recently, electrochemical cells have been actively studied. Among the electrochemical cells, for example, a fuel cell is a system in which a fuel such as hydrogen is electrochemically reacted with an oxidant such as oxygen to generate electric power. Among them, a polymer electrolyte membrane fuel cell (PEFC) can operate at low temperatures as compared with other fuel cells and its reaction product is water, thereby reducing an environmental load. Hence this cell has been supplied to practical use as a household stationary power source, and further as an automotive power source. For the full-scale spread of PEFC, it is necessary to greatly reduce the amount of a noble metal catalyst contained in the catalyst layer of each electrode.
Generally, a carbon-supported catalyst in which a noble metal catalyst material is supported by a carbon black support is used for a catalyst layer of PEFC. When the carbon-supported catalyst is used as the automotive power source, the carbon support contained in the catalyst layer is corroded by the start and stop operation. Further, the catalyst itself supported by the carbon support is dissolved. It has been reported that this accelerates deterioration of the catalyst layer and a membrane electrode assembly (MEA) containing the catalyst layer. Development of a noble metal catalyst layer having high durability and a high reaction area is essential to greatly reduce the amount of the noble metal catalyst. A catalyst layer formed by sputtering or vapor-depositing the catalyst material is carbonless. Accordingly, deterioration due to corrosion of a catalyst support can be avoided.
In order to achieve a high reaction area, there has been suggested a method comprising: forming a catalyst layer precursor containing a porogen (pore-forming material) and a noble metal catalyst material by sputtering or vapor-depositing; and removing the porogen to produce a catalyst layer containing pores. However, the reaction area of the noble metal catalyst is not still sufficient. If a large amount of the porogen is introduced into the catalyst layer to increase the porosity, many large cavities with a size of more than 100 nm appear in the catalyst layer. This causes deterioration of fuel cell performance.